By way of background to the present invention, FIG. 1 and FIG. 1A show a prior art horizontal axis wind turbine 10 comprising a tower 11, a rotor 12 and a nacelle 13. The nacelle 13 is supported at an upper end 14 of the tower 11, which can be 100 meters above ground level 15 for a multi-megawatt turbine. The nacelle 13 houses the rotor shaft, generator and gearbox (if present). A wind-monitoring device 16 is located on top of the nacelle 13. The wind-monitoring device 16 includes an anemometer 17 for monitoring wind speed and a vane 18 for monitoring wind direction in a horizontal plane.
The rotor 12 is supported by the nacelle 13 and includes three rotor blades 19a, 19b, 19c. Only two blades 19a, 19b are shown in the side view of FIG. 1. However, all three blades 19a, 19b, 19c can be seen in the front view of FIG. 1A. Referring to FIG. 1A, the blades 19a, 19b, 19c are equally spaced about a central rotor hub 20, and extend radially from the hub 20 when viewed from the front, in a span direction from root 21 to tip 22. As the rotor 12 rotates, the tips 22 of the blades 19a, 19b, 19c sweep a circular area 23 known as the ‘rotor disc’, which is represented by a dashed line 24 in FIG. 1A, and a dashed vertical line 24 in FIG. 1.
Referring again to FIG. 1, the rotor 12 is mounted upwind of the tower 11 and faces directly into the wind, which is represented by arrows 25. The wind turbine 10 is therefore known as a horizontal axis ‘upwind turbine’. The rotor 12 extracts energy from the wind as it rotates. This causes a reduction in wind speed downstream of the rotor 12. The area of reduced wind speed is commonly referred to as the ‘wake’ of the rotor 12. The wake spreads out with increasing distance from the rotor 12, and is represented by the dotted lines 26 in FIG. 1. The wind turbine 10 includes a yaw mechanism (not shown) for turning the nacelle 13 about a vertical yaw axis 27 to keep the rotor 12 facing into the wind with changing wind direction.
The wind turbine 10 typically includes a pitch mechanism (not shown), located within the rotor hub 20, for turning or ‘pitching’ the blades 19a, 19b, 19c about their longitudinal axes 28. Pitching the blades 19a, 19b, 19c varies the angle of attack (and hence the lift) of the blades 19a, 19b, 19c, which allows the rotor speed to be maintained within predefined operating limits despite changing wind speeds. In addition to controlling rotor speed, blade pitching is used to smooth out variations in loading and torque as described below.
The blades 19a, 19b, 19c of the rotor 12 experience significantly different wind velocities at different points within a rotational cycle because wind speed generally varies with height. For example, the upwardly-extending blade 19b in FIG. 1 may experience a faster wind speed and hence greater lift than the downwardly-extending blade 19a. To compensate for differences in wind speed with height, some modern wind turbines employ ‘cyclic pitch control’ to vary the angle of attack of the blades continuously during a rotational cycle. So, for example, the angle of attack of a blade may be increased as the blade passes the tower to increase the lift generated by a downward-pointing blade 19a. Cyclic pitching ensures that the blades provide substantially the same lift at all points in the rotational cycle, such that the blades 19a, 19b, 19c are exposed to substantially the same flapwise bending moments during a rotor cycle and fatigue and extreme loads on the complete wind turbine system are reduced.
The wind turbine 10 includes a wind turbine control system which, amongst other things, controls the yaw and pitch mechanisms. The wind turbine control system includes a controller 29 that receives signals indicative of wind speed from the anemometer 17, and wind direction from the vane 18, and calculates the required variations for yaw and pitch. A model linking the estimated variation of wind speed and direction with height is employed to determine the requisite parameters for cyclic pitch control based upon the wind-speed readings from the anemometer 17.
Whilst existing wind turbine control systems work well, there is a continual drive to produce more sophisticated control systems and control strategies. Indeed, it is an aim of the present invention to provide a more sophisticated wind turbine control system that better handles some other technical challenges that will now be explained.
The blades of modern wind turbines are inherently flexible and can bend significantly in use. As the blades are long, in excess of 50 meters in many cases, any flexing or bending of the blades may translate into considerable displacement of the tips of the blades out of the rotor disc in the wind direction. For upwind turbines, wind loading will tend to force the blades towards the tower. Flexing of the blades in this way presents a risk of a collision between the blades and the tower in extreme conditions. The risk is greatest in cases of ‘negative’ wind shear, i.e. when the wind speed is higher in the lower part of the rotor disc than in the upper part.
In order to prevent the blades from colliding with the tower, modern wind turbines are designed to ensure that the clearance between the tip of the blade and the tower, i.e. the ‘tip-to-tower’ distance (represented by the double-headed arrow 30 in FIG. 1), remains within predefined safe limits. To this end, several measures are presently employed:                Firstly, the blades 19a, 19b, 19c may be pre-bent so that they curve away from the tower 11 moving in the span direction from root 21 to tip 22, but straighten when under load. Without pre-bending, the tips 22 of the blades 19a, 19b, 19c could bend undesirably close to the tower 11 when under load.        Secondly, the nacelle 13 and rotor 12 are tilted as shown in FIG. 1, such that a rotor axis 31 about which the rotor 12 turns is inclined upwardly into the wind with respect to a horizontal axis 32. The extent of tilt is defined by a ‘tilt angle’, which is the angle between the rotor axis 31 and the horizontal 32. Tilting the rotor 12 in this way increases the clearance between the tower 11 and the tips 22 of the blades 19a, 19b, 19c.         Thirdly, the blades 19a, 19b, 19c are inclined dihedrally in the span direction away from the tower 11 moving from root 21 to tip 22. This is known as ‘coning’ because the blades sweep a cone-shaped area. The extent of coning is defined by a ‘cone angle’, which is the angle between the longitudinal axis 28 of the blades 19a, 19b, 19c along the span direction, and the rotor disc 24. When the rotor 12 is not tilted, the cone angle is simply the angle between the longitudinal axis 28 of a blade 19a, 19b, 19c and the vertical yaw axis 27 about which the nacelle 13 turns. It should be noted that the extent of coning shown in FIG. 1 has been greatly exaggerated to improve understanding of the present invention.        Fourthly, the blades 19a, 19b, 19c incorporate structural features to increase their rigidity and reduce their propensity to bend towards the tower 11 to an extent beyond that desirable to compensate for any pre-bending.        
Excessive pre-bending, tilt and coning can reduce the performance and hence reduce the efficiency of the wind turbine. In addition, excessive pre-bending can make blades difficult to manufacture and problematic to transport. Furthermore, increasing the rigidity of the blades generally means increasing the weight of the blades and hence increasing the size and weight of other wind turbine components that must support the heavier blades. This increases the cost of the wind turbine and may further reduce its performance and efficiency.